Electrochemical gas sensor system with improved accuracy and speed

ABSTRACT

An electrochemical cell for sensing gas has added mechanical support for the working electrode to prevent flexure of the working electrode due to pressure differentials. The added mechanical support includes: 1) affixing a larger area of the working electrode to the body of the cell; 2) a gas vent to a cavity of the body to equalize pressures; 3) a rigid electrolyte layer abutting a back surface of the working electrode; 4) infusing an adhesive deep into sides of the porous working electrode to enhance rigidity; 5) supporting opposing surfaces of the working electrode with the rigid package body; and 6) other techniques to make the working electrode more rigid. A bias circuit is also described that uses a controllable current source, an integrator of the varying current, and a feedback circuit for supplying a voltage to the counter electrode and a bias voltage to the reference electrode.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is based on and claims priority from U.S. provisionalpatent application Ser. No. 62/459,597, filed on Feb. 15, 2017, by JimChih-Min Cheng et al., assigned to the present assignee and incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to the sensing and identification of gasses by anelectrochemical cell in conjunction with a sensing circuit and, inparticular, to techniques for reducing flexure of a working electrodeand an improved biasing circuit.

BACKGROUND

Given the dramatic changes in the earth's atmosphere, precipitated byindustrialization and natural sources, as well as the dramaticallyincreasing number of household and urban pollution sources, the need foraccurate and continuous air quality monitoring has become necessary toboth identify the sources and warn consumers of impending danger.Tantamount to making real-time monitoring and exposure assessment areality is the ability to deliver, low cost, small form factor, and lowpower devices which can be integrated into the broadest range ofplatforms and applications.

There are multiple methods of sensing distinct low density materialssuch as gasses. Common methods include gas chromatography, nondispersiveinfrared spectroscopy (NDIR), the use of metal oxide sensors, the usechemiresistors, and the use of electrochemical sensors. The presentinvention pertains to electrochemical sensors. The principle ofoperation of an electrochemical sensor is well known and is summarizedin the following overview:http://www.spec-sensors.com/wp-content/uploads/2016/05/SPEC-Sensor-Operation-Overview.pdf,incorporated herein by reference.

Basically, in an electrochemical sensor, a porous sensor electrode (alsoknown as a working electrode) contacts a suitable electrolyte. The gaspermeates the electrode and contacts the electrolyte. The sensorelectrode typically comprises a catalytic metal that reacts with thetarget gas and electrolyte to release or accept electrons, which createsa characteristic current in the electrolyte when the electrode isproperly biased and when used in conjunction with an appropriatecounter-electrode. The current is generally proportional to the amountof target gas contacting the sensor electrode. By using a sensorelectrode material and bias that is appropriate for the particular gasto be detected, the concentration of the target gas in the ambientatmosphere can be determined from the sensing current.

The sensitivity of an electrochemical cell to a particular gas may beimpacted by the application of a bias voltage to that cell. Therefore,by applying a set of different biases to an electrochemical cell, andcomparing the recorded signal to a library of signals (in a look-uptable) corresponding to those signals characteristic of individual knowngasses, it is possible to ascertain the presence of, differentiatebetween, and quantify the occurrence of multiple gasses in theenvironment of the sensor. By continuously and quickly ramping the biasapplied to the cell, a single electrochemical cell may rapidlydifferentiate between multiple gasses in its environment. Identificationof an analyte in a controlled environment, such as that of a laboratory,via a potential ramping scheme is known in the art as “voltammetry”.However, electrochemical sensing systems known in the art are limitedwith respect to the speed at which voltammetry may run due to acombination of the electrochemical cells having largeresistance-capaciotance (RC) time constants and the drive electronicshaving long settling times, resulting in voltammetry measurements takingup to 20 minutes or even more. Ramping the bias voltage at only a fewmillivolts per second is typical.

In the event that such a measurement is performed in a highly controlledenvironment, such a long data collection time is of no consequence sincethe test conditions may be controlled over that timeframe. However,outside of a controlled laboratory environment, the ambient conditionsof the cell may change significantly over the course of such an extendedperiod of data collection. For example, in an everyday consumer usecase, the ambient relative humidity, the ambient temperature, and theambient mixture and concentrations of analyte gasses may changesignificantly over such an extended data gathering period. All thesefactors combined with electronic drift inherent to running voltammetryon the electrochemical cell render analysis of the data meaningless.Further, if the gas sensor is intended detect dangerous gasses, the longdelay time may result in a harmful effect.

Accordingly, for such a consumer application, an electrochemical gassensor is required having small form factor, and hence a small timeconstant, such as described in the Applicant's U.S. patent applicationSer. No. 15/598,228, in conjunction with a novel drive circuit enablingshort settling times. Such a system enables voltammetry to be performedin a period of a few seconds or less. Over such a timescale, the ambientconditions in a majority of uncontrolled consumer environments in whichthe cell may be present would be essentially invariant, resulting in theability to perform voltammetry in an essentially uncontrolledenvironment whilst providing data of sufficient quality to enableaccurate electrochemical analysis.

The varying bias voltage may be sinusoidal or have another waveform. Itis common to supply the variable bias voltage via a digital-to-analogconverter (DAC) that outputs a varying voltage. Such an output containsdiscrete steps due to the quantized nature of the DAC output, and thusthere is a high dv/dt. The rate at which the bias may be ramped in anelectrochemical cell is limited, among other factors, by the capacitanceof the cell, and the presence of current spikes occurring within thecell during the step- wise application of the bias ramp to the cell. Theimpact of the capacitive nature of the cell on the current spikesoccurring during a voltage step is described by the equation i=C dv/dt.Excessive transient currents can lead to damage of the cell. In certainschemes, accurate measurement of the cell requires for the transientcurrents to have mostly decayed so that the cell output is measuredwhile in its steady-state condition. Accordingly, a settling timetypically occurs between the point at which a voltage step is applied tothe cell and the point at which the current generated at the workingelectrode of the cell is measured. This limits the rate at which a biassweep of the cell may be applied. Minimizing the capacitance of the celland occurrence of current spikes within it allows for maximizing therate at which voltammetry may be performed, hence maximizing performanceof the sensor.

Electronic noise is also an important issue in electrochemical sensorswhich affects accuracy and speed achievable by the sensors.Electrochemical sensors are known for picking up 60 Hz and a variety ofRF noise due to the large electrodes and electrolytes. Commercialsystems can minimize the noise through shielded housings around thesensors, powering the sensor using a battery, and using signalprocessing to filter out the noise. However, noisy environments stillgenerally limit the accuracy of the electrochemical systems and size ofthe systems due to the extra electronics needed to minimize the noise.This impacts the rate at which the ramps of the voltammetry can beexecuted at due to higher current spikes and longer settling times whichcan occur during step transitions due to the additional circuitelements, such as resistors and capacitors, which need to be added tomitigate noise.

Additionally, any flexure of the working electrode changes itselectrical characteristics. Flexure may occur due to gas pressurefluctuations. Pressure built up due to small pore size in the electrodecan also change the position/formation of the three phase interfaceinside the electrode which can impact the sensitivity of the sensor. Thethree phase interface is a critical aspect of a gas diffusion electrodeand is formed at the conjunction interface of a gas, solid, and liquid(i.e., gas/electrode/electrolyte). A change in pressure could influencethe density of the three-phase interface and where the three-phaseinterface is formed.

Accordingly, what is needed is an electrochemical sensor system forgasses that employs a varying bias signal that does not have high dv/dtcharacteristics. What is also needed is an electrochemical sensor thatis less susceptible to noise. Additionally, an electrochemical sensorwhere the working electrode does not substantially flex due to changinggas pressures is required for stability in changing environments.Further, a control method is desired for use with a low capacitance gassensor that can quickly detect a variety of different gasses in a shorttime.

SUMMARY

A sweeping bias voltage generator is disclosed that applies a varyingbias voltage to an electrochemical sensor, where the bias voltage may besinusoidal or ramping without any steps, so is continuous. Therefore,current spikes are negligible due to i =C dv/dt. As a result, little orno settling time is needed, and the sensor can have a very rapidresponse to changing ambient gasses.

To minimize flexure of the working electrode, mechanical supportfeatures of the working electrode are disclosed that prevent the workingelectrode being distorted due to changing gas pressures. Additionally, asensor structure is described that equalizes the gas pressure on theopposing surfaces of the working electrode to prevent flexure.

To minimize electronic noise, conductors and metal shielding areembedded in the walls of the sensor body (e.g., molded ceramic), whichprevent impact of noise to the system. Embedding the conductors andshielding protects the metal and prevents corrosion due to electrolyte(e.g., acid) contact and environmental exposure. This also allows forsimplification of circuit design, allows for minimal integrated circuitsize (so the circuit can be integrated with the sensor), and allows forsimplification of post- processing due to less noise.

Uses of the sensor module include detection of air quality (e.g., carbonmonoxide), gas exposure control, toxic gas detection, breath analysis,feedback in industrial processes, etc.

Other embodiments and advantages are described.

BRIEF DESCRIPTIONS OF DRAWINGS

FIG. 1 illustrates a bias circuit coupled to an electrochemical cell inaccordance with one embodiment of the invention.

FIG. 2 is a cross-section of a portion of an electrochemical cell wherethe working electrode (WE) is affixed to the rigid body of the cell overa large area of the WE to prevent flexure of the WE.

FIG. 3 is a cross-section of a portion of an electrochemical cell wherea rigid electrolyte layer mechanically supports the back surface of theWE to prevent flexure of the WE.

FIG. 4 is a cross-section of a portion of an electrochemical cell wherean incoming gas is allowed to pressurize a cavity in the body toequalize the pressures on opposing surfaces of the WE.

FIG. 5 is a cross-section of a portion of an electrochemical cell wherea rigid electrolyte layer mechanically supports the back surface of theWE to prevent flexure of the WE, where the sides of the WE are sealed toprevent the gas exiting from the sides of the porous WE, and where thesides of the counter electrode (CE) are also sealed to prevent gas fromentering the sides of the CE.

FIG. 6 is a cross-section of an electrochemical cell where the rigidpackaging body itself is used to mechanically support the back surfaceof the WE so as to prevent flexure.

FIG. 7 is a cross-section of an electrochemical cell with an integratedshield layer and conductors for routing signals to the electrodes. Theshield layer and conductors are completely encased within walls of thepackage body. The conductors terminate in metal pads for bonding to aprinted circuit board.

Elements that are the same or equivalent in the various figures arelabeled with the same numeral.

DETAILED DESCRIPTION

FIG. 1 illustrates an electrochemical sensor module 10, showing a cell12 and a bias circuit 14 for sweeping a bias voltage to allow the cell12 to detect different types of gasses. The cell features are designedso that the cell 12 can detect different types of gasses in response todifferent bias voltage levels. The voltage should be swept as rapidly aspractical to more quickly identify the presence of a target gas, whichmay be a harmful gas.

Basically, the cell 12 contains a cavity 16 containing an electrolytethat is in contact with a working electrode (WE) 18, a counter electrode(CE) 20, and a reference electrode (RE) 22. Additional electrodes can beadded as additional working and counter electrodes for differentvariants of the cell for gas detection. The RE 22 provides a means bywhich a reference potential may be applied to the cell.

The body of the cell 12 includes a gas opening to allow diffusion of thegas or atmosphere being sensed into the porous WE 18. In certainembodiments, the opening is partially or fully filled with a porousmaterial which allows gas to diffuse into the WE 18 but blocks theliquid or paste-like electrolyte from exiting the cavity.

The electrodes 18/20/22 may comprise an electrically conductingmaterial, such as carbon, and a catalyst such as ruthenium, copper,gold, silver, platinum, iron, ruthenium, nickel, palladium, cobalt,rhodium, iridium, osmium, vanadium, or any other suitable transitionmetal and their alloys. The catalyst is selected so as to react with oneor more particular gases. The electrodes 18/20/22 may be partiallypermeable to both the electrolyte and the gas to be detected so that theelectrochemical reaction may occur within the body of the electrodes18/20/22.

The electrolyte may comprise an ionic material such as an acid. Theelectrolyte may be viscous such as a gel, or may be a polymer infusedwith an organic or inorganic acid.

The bias circuit 14 applies electrical potentials between the WE 18, CE20, and RE 22. A sensing circuit 26 senses electrical currents passingbetween the WE 18, CE 20, and RE 22, and reports on the sensed signals.The sensing circuit 26 may comprise a trans-impedance amplifier (TIA) inconjunction with an analog-to-digital converter (ADC) capable ofconverting the sensed signal from the working electrode into a digitalrepresentation. The digital signals are processed by a microprocessor onwhich algorithms may be stored and executed enabling, for example,reporting out of calibrated gas concentrations. The sensing circuit 26and its connections to the electrodes may be conventional.

In an example, when a toxic gas such as carbon monoxide (CO) comes incontact with the WE 18 (the sensing electrode), oxidation of CO gas willoccur on the WE 18 through chemical reaction with water molecules in theair. Connecting the WE 18 and the CE 20 through a potentiostat circuitwill allow protons (H+) generated on the WE 18 to flow toward the CE 20through the electrolyte (an ion conductor). In addition, generatedelectrons move to the CE 20 through the potentiostat circuit. A reactionwith oxygen will occur on the CE 20 to reform water. By measuring thelevel of current between the WE 18 and the CE 20, the electrochemicalcell 12 can detect the concentration of the target gas (e.g., CO) in theair. This is usually done at a single bias point. Different types ofelectrodes and different bias voltages traditionally have been used todetect different types of gases.

FIG. 1 illustrates a bias circuit (a potentiostat) for use with anelectrochemical cell, in accordance with one embodiment of theinvention. The bias circuit supplies a variable bias signal to the RE22.

A digital control signal is applied to the input of a current source 28that converts the digital code to an analog current. The digital codemay be constant or vary. A varying digital code may cause the currentsource 28 to output a substantially sinusoidal current. The current isapplied, via an input resistor 30, to the inverting input of an op amp32. The op amp 32 is connected as an integrator due to the feedbackcapacitor 34 being connected between the output and the inverting input.The non-inverting input is connected to ground. The feedback operates tocause the potential between the inverting and non-inverting inputs toapproximately equal zero volts. The integration of the input currentresults in a continuous signal at the output of the op amp 32, even ifthe output of the current source 28 may be stepped. The output of the opamp 32 varies continuously as a function of the current output by thecurrent source 28. Supplying a positive current by the current source 28decreases the potential at the output of the op amp 32, and providing anegative current increases the potential at the output. Providing zerocurrent maintains a constant output voltage. The output of the op amp 32is a reverse integral (180° phase shift of the integral) of the inputcurrent. Step functions at the input do not cause discontinuities in theoutput voltage of the op amp 32.

The continuous output of the op amp 32 is then applied to the invertinginput of an op amp 38, via a resistor 40, whose output is coupled to theCE 20. The non-inverting input of the op amp 38 is coupled to ground,and the feedback loop tries to keep the potential between the invertingand non-inverting inputs at zero volts.

The RE 22 is coupled to the non-inverting input of another op amp 44,whose inverting input is coupled to its output. The output of the op amp44 is coupled to the inverting input of the op amp 38 via a resistor 46,the non-inverting input of the op amp 38 is connected to ground, and thefeedback loop tries to keep the potential between RE 22 and theinverting input of op amp 38 at zero volts.

The op amp 44 is a unity gain amplifier. To the first order, the op amp44 and its feedback loop have no impact on the circuit. In reality, opamps are non-ideal and have input leakage current. Input leakage currentis typically larger into the inverting input of an op amp than it isinto the non-inverting input. The role of op amp 44 is to reduce theinput leakage current drawn from the RE 22. The RE 22 ideally applies abias to the electrochemical cell, but does not draw a current. If itdoes so, it impacts the performance and state of the cell. Thepotentiostat circuit (based on op amp 38) works by changing the bias andcurrent being driven through the CE 20 such that: (i) the RE 22 is heldat the bias potential being applied at the inverting input to op amp 38,and (ii) the current required to support the electrochemical reactionoccurring within the cell between the WE 18 and the CE 20 is provided tothe cell.

Accordingly, the RE 22 of the electrochemical cell 12 is properly biasedat a varying (e.g., ramping) continuous signal for operating the gassensor. The RE 22 can be biased at any voltage. The circuit allows thepotential of the CE 20 to vary as required to enable the requiredcurrent to flow between the CE 20 and the WE 18. This is known asvoltammetry. Suppression of discontinuities in the applied bias enablesthe sensor to be accurately controlled with minimized or completelyeliminated transient signals that could negatively affect performance.The ramping may be performed rapidly to quickly detect a range of gassesin an uncontrolled environment. No settling time is needed. If adangerous gas is detected, circuitry may be used to generate an alarm orturn off the source of the gas.

A capacitor (not shown) may optionally be coupled between the RE 22 andthe CE 20 to filter high frequency signals.

The op amp 44 may be deleted by connecting a resistor divider betweenthe RE 22 and the inverting input of the op amp 38, with a capacitorconnected between the resistor nodes and the CE 20; or in parallel withthe resistor divider.

There are various ways to detect the reaction of the sensor to thetarget gasses. The gas diffuses into the porous WE 18 where it isoxidized or reduced. This electrochemical reaction results in anelectric current that passes through the external sense circuit 26. Thebias circuit 14 maintains a bias voltage between the WE 18 and RE 22. Atthe CE 20, a counter half reaction occurs such that the electronsgenerated or consumed at the WE 18 equals the electrons generated orconsumed at the CE 20. Additionally, should the CE 20 be exposed to theenvironment instead of the WE 18, the CE 20 will run the forwardreaction while the WE 18 will run the reverse reaction. In gas sensing,it is rare for the CE 20 to face the environment, though such aconfiguration is known to be possible.

The magnitude of the current is controlled by how much of the arget gasis reacted at the WE 18 so that the output from the sensor is linearlyproportional to the gas concentration. Accordingly, the sense circuit 26measures the current and may perform an analysis to associate thecurrent to the bias voltage and to the target gas being presentlymeasured.

Since the bias circuit 14 is current-controlled rather than voltagecontrolled, there are no current spikes due to capacitance (so notransient settling time) and the sensor can operate to rapidly detectthe presence of various target gasses.

In one embodiment, the current source 28 and the electrical componentsare selected to vary the applied bias between 1V and −1V. The biasvoltages may also be greater than 1V, such as up to 2.5V, and less than−1V, such as down to −2.5V. Some target gases, such as ethylene oxide,may require even higher bias levels for its oxidation depending on theelectrode/electrolyte combination chosen, while some gases, such ascarbon monoxide, only require a relatively low bias level. Some gaseseven oxidize at multiple bias levels. The two gases may bedifferentiated based on the varying bias levels needed to react thegasses at the WE 18. In more convoluted cases where the two gases havevery similar bias levels, a more detailed voltammogram can be created bysweeping a number of bias points.

The bias circuit 14 and sensor 26 may be formed as a single integratedcircuit.

The entire sensor may use a footprint of less than 10 mm×10 mm.

A novel drive scheme is also disclosed. To identify a single gas, ormultiple mixed gasses, multiple biases are applied to the RE 22. Sincedifferent reactions are promoted or suppressed at different appliedbiases, it is possible to identify a particular gas by looking at the“spectrum” of the gas registered during the application of multiple biaspoints.

In the case of multiple gasses being present, in theory, the recordedspectrum of the gas mixture comprises the linear summation of thespectra of the individual component gasses, scaled by their relativeconcentrations and sensitivities. By comparing the captured spectrum toa reference library of gas spectra, it should be possible to identifythe multiple gasses present.

Similar gasses will have similar peaks in the recorded spectra. Forexample, methanol and ethanol comprise similar chemical bonds, hence mayhave similar spectra which may have substantial overlap during any oneparticular bias sweep. Whereas it is known in the art that changing thewaveform of an applied bias to an analyte may change the position of therecorded peaks, it is typically difficult to practically implement dueto the limited bias ramp rates which may be successfully applied totypical electrochemical systems. This principle is known for liquidspectroscopy applications, though to the best of the inventors'knowledge, not yet demonstrated for electrochemical spectroscopy forgases due to the limited hardware. Accordingly, in the case of anelectrochemical cell enabling fast ramping of an applied bias, it ispossible to greatly further differentiate between the two or moresimilar gasses by changing the applied bias waveform and especially thespeed of bias ramp, dV/dt over and above that capability in standardelectrochemical systems.

In addition to being able to determine the presence of chemicals,voltammetry enables the determination of the relative humidity, thetemperature, and the ionic state of the electrochemical cell. Thesefactors are known to impact the sensitivity of electrochemical cells togasses the cell is exposed to. In standards and the current state-of-the-art, often one or more of the factors are bundled into thegeneric parameter of drift resultant of limited cell hardware andability to generate the necessary waveforms for proper compensation. Inthe case of voltammetry performed in an uncontrolled environment but inthe order of a few seconds or less and in which the ambient and internalstate of the cell are therefore essentially constant, it is possible todetermine the relative humidity, temperature and ionic state of theelectrochemical cell at the time at which the cell is also being exposedto a gas. This data can therefore be used to compensate for the impactof these factors on the sensitivity of the cell to a gas, therebyimproving the accuracy with which the system can identify and quantifygasses.

Data collected during calibration of the sensor in a controlledenvironment is stored in a local look-up table, such as located in thesame integrated circuit as the bias circuit and microprocessor. Sensorcharacteristics (bias levels, sensor current, etc.) corresponding toknown target gasses and environmental conditions are also stored.Changes in the detected environment relative to the calibration data arethen used to determine the concentration of targeted gasses in theenvironment and other environmental factors as the bias voltage isswept. The gas readings may be compensated by the detected humidity,temperature, etc. to determine the concentration. Rather than the biasbeing swept over the prior art cycle time of, for example, 20 minutes ormore, the cycle time using the present invention may be less than oneminute. This enables the rapid detection of a variety of gasses during astable environment.

The waveforms corresponding to the detection of target gasses in theenvironment may comprise overlapping gas signals. A microprocessor thendifferentiates the different gasses corresponding to the overlapping gassignals based on stored data and algorithms. The microprocessor mayperform as an inference engine with a set of equations that identifygasses present.

Another feature of the sensor relates to techniques that reduce oreliminate any flexure of the WE 18. Flexure of the WE 18 in the presenceof changing conditions will change the response of the WE 18 to thetarget gasses. Calibration is initially performed under idealconditions, and the sensor readings will be skewed if they occur whilethe WE 18 is flexed relative to it calibration state. One cause offlexure is a gas pressure difference on the front and back surfaces ofthe WE 18.

FIG. 2 is a cross-sectional view of a portion of an electrochemicalcell. A WE 60 is shown affixed to the rigid body 62 (e.g., ceramic) ofthe cell by an adhesive 63. The cell may be only a few millimeters inwidth and length. An opening 64 in the body 62 allows ambient gas tocontact the WE 60. The WE 60 is porous, and the gas permeates through toan electrolyte (not shown) in contact with the back surface of the WE60. The electrolyte carries charges between the CE (not shown) and theWE 60 to generate the currents that effectively identify the presence ofthe target gas.

The adhesive 63 also acts as a gas seal. The adhesive 63 may be epoxy,silicone, acrylate, or other adhesive, and may be conducting, forexample via the inclusion of conducting metal, carbon, or otherparticles. If the adhesive 63 is conductive, it may serve toelectrically connect the WE 60 to conductors that lead to metal pads forbonding to a printed circuit board. The entire front surface of the WE60 except for the gas opening 64 area is affixed to the rigid body 62,so flexure of the WE 60 due to temperature fluctuations and pressuredifferentials is prevented. The WE 60 is also formed to be rigid toprevent flexure under normal operating conditions. The gas diffuseslaterally and into the WE 60, so it is important that the adhesive 63does not substantially diffuse into the WE 60. The adhesive 63 may beapplied by printing or stamping onto the body 62. The electrolyte may bea liquid or gel contacting the back surface of the WE 60 so does notprovide substantial mechanical support to the WE 60.

FIG. 3 is a cross-sectional view of a portion of another electrochemicalcell where a WE 70 and a CE 72 are affixed to the rigid body 74 of thecell via an adhesive 76. The adhesive 76 may be conducting to connectthe electrodes to metal pads on the body 74. An opening 78 in the body74 allows ambient gas to diffuse into the WE 70. Unlike in FIG. 2, theadhesive 76 is only applied to a perimeter portion of the WE 70. Thisallows the gas to enter a small sealed cavity between the WE 70 and thebody 74 to diffuse into a larger area of the WE 70 sooner for a fasterreaction time. The adhesive 76 blocks the gas from contacting the CE 72.Thus, the adhesive 76 itself does not substantially inhibit flexure ofthe WE 70.

The flexure of the WE 70 and CE 72 is inhibited by a fairly rigidelectrolyte layer 80 that contacts the back surfaces of the WE 70 andthe CE 72. The electrolyte layer 80 may be a rigid but porous polymerthat is infused with an ionic material, such as sulfuric acid orphosphoric acid. The electrolyte layer 80 is sufficiently thick andrigid to prevent flexure of the WE 70 and CE 72 under normal operatingconditions. Charges are free to move between the WE 70 and CE 72 throughthe electrolyte layer 80 to create the current signature of the targetgas under the proper biasing conditions. The electrolyte layer 80essentially forms an ion bridge between the electrodes. The electrolytelayer 80 may diffuse up to 10 microns or more into the porous WE 70 andCE 72 for good contact. Note that there may be a pressure differentialbetween the gas on opposing surfaces of the WE 70, and such a pressuredifferential is not sufficient to flex the WE 70 due to the rigidity ofthe electrolyte layer 80.

FIG. 4 is a cross-sectional view of a portion of an electrochemical cellwhere the back and front surfaces of the WE 82 are exposed to the samegas pressure due to an opening 84 in the adhesive 86. The adhesive 86may extend around almost the entire perimeter of the WE 82 for goodmechanical support, but just needs to have a small opening to equalizethe gas pressures. The body 88 may be ceramic. Since the target gas 89enters the back cavity 90 of the body 88, the CE 92 must be sealed fromthe back cavity 90 with an adhesive or sealant 94, such as epoxy. Anelectrolyte layer 96 may be a porous material infused with anelectrolyte, such as an acid, for transporting charge between the WE 82and the CE 92. The electrolyte layer 96 may be sufficiently non-porousto gasses so as to prevent the gas 89 in the back cavity 90 fromdiffusing into the electrolyte and reacting with the back surface of theWE 82. If such “back” diffusion took place, the sensor would take a longtime to react to changes in the ambient air.

In a variation of FIG. 4, the bottom surface perimeter of the WE 82 iscompletely sealed by the adhesive 86, and the gas 89 is allowed todiffuse laterally through the highly porous WE 82 and exit an edge ofthe WE 82 to enter the back cavity 90 of the body 88. A drawback of thisdesign is that the back pressure is slow to react to a changing frontpressure. The CE 92 has its sides sealed by the sealant 94 to preventthe gas from diffusing out the side of the WE 82 and reacting with theCE 92.

FIG. 5 is a cross-sectional view of a portion of another electrochemicalcell where the sides of the porous WE 100 are sealed by a sealant 102,such as epoxy, to prevent the gas from exiting out the sides andaffecting other electrodes. The CE 92 also has its sides sealed by thesealant 94, as described with respect to FIG. 4. The side sealants maybe applied by needle dispense, jetting, printing, or other method priorto or after the electrodes are bonded to the body 88. Alternatively, theside sealants may be applied to the electrodes prior to theirsingulation, such as by printing. In this case, the electrode sides aresealed prior to being bonded to the body 88 and no further side sealingis required. Finally, the electrodes can be placed in individualcavities to isolate them from each other. This cavity-based separationprovides alternative methods to increase structural support of theelectrodes.

Sealing the sides of the electrodes also prevents fraying of theelectrodes. The sealant may be any suitable polymer.

FIG. 6 illustrates another embodiment of a way to mechanical stabilizethe WE100 to prevent flexure. FIG. 6 may be identical to FIG. 5 but therigid body 88, such as a ceramic, directly contacts the back surface ofthe electrolyte layer 96, which may also be rigid like the electrolytelayer 80 in FIG. 3. This, in turn, supports the WE 100 in contact withthe front surface of the electrolyte layer 96. Therefore, the rigidpackaging itself is used to mechanically support the back surface of theWE 100 so as to prevent flexure.

In all embodiments, there may be separate cavities in the body 88 whichcontain at least a working electrode and a counter electrode. Eachcavity and associated electrodes is independent and may be biased for aparticular gas. All the cavities receive the ambient gas and each cavitymay be dedicated to a particular target gas. A current sensor andmicroprocessor may use multiplexing to detect the various gasses sensedby the different cavities.

FIG. 7 is a cross-section of an electrochemical cell with an integratedshield layer and conductors for routing signals. The dielectric body105, such as ceramic, is molded around a leadframe forming conductors104 that conduct the sensing current and the bias voltages. Theconductors 104 are complexly encased by the body material and runbetween the inner and outer walls of the body 105. The adhesive 86 thatseals the gas and affixes the electrodes to the body may be a conductiveadhesive and electrically contacts the WE 100. Other conductors areelectrically connected to the CE 92 outside of the drawing. All theconductors 104 along each surface of the body 105 may be in the sameplane. The conductors 104 are exposed at the top of the body 105 to formmetal pads 106 for bonding (such as soldering) to a circuit board. Anynumber of pads 106 may be used.

The sensing current has a small signal-to-noise ratio, so any noise onthe conductors 104 is significant. The conductors 104 are shielded fromnoise by a metal shield layer 108, which may be grounded or floating.The shield layer 108 is encased in the body 105. The shield layer 108may be located around the top, bottom, and sides of the body 105,depending on the shielding requirements, to substantially surround theconductors 104. The shield layer 108 greatly reduces external noisebeing coupled to the bias signals and sensor current signals.

By embedding the conductors 104 within the body 105, they are protectedfrom the electrolyte (e.g., a corrosive acid) and other possible damage.This allows the conductors 104 to be formed of virtually any type ofhighly conductive metal and allows the conductors 104 to be efficientlyrouted within the body 105.

While particular embodiments of the present invention have been shownand described, it will be obvious to those skilled in the art thatchanges and modifications may be made without departing from thisinvention in its broader aspects and, therefore, the appended claims areto encompass within their scope all such changes and modifications thatare within the true spirit and scope of this invention.

What is claimed is:
 1. An electrochemical gas sensor comprising: a bodycontaining one or more cavities; an electrolyte contained within the oneor more cavities; a plurality of electrodes on the inside of the one ormore cavities, the electrodes being in contact with the electrolyte, theelectrodes including a porous first electrode; and a gas opening in thebody for allowing a gas to enter the first electrode to cause a chemicalreaction to occur, the first electrode being supported within the cavityto substantially prevent flexure of the first electrode due to changinggas pressures.
 2. The sensor of claim 1 further comprising an adhesivesubstantially covering an entire first surface of the first electrodeexcept for a gas inlet area for rigidly affixing the first electrode tothe body.
 3. The sensor of claim 1 further comprising a gas passagewaythat allows the gas that enters the first electrode via a front surfaceof the first electrode to also enter the cavity and provide a gaspressure to an opposing back surface of the first electrode tosubstantially equalize gas pressures applied to the front surface andthe back surface.
 4. The sensor of claim 3 further comprising anadhesive affixing the first electrode to the body, the adhesive beingpatterned to provide the gas passageway into the cavity.
 5. The sensorof claim 3 wherein the first electrode is configured to allow the gas todiffuse out of a side of the first electrode and into the cavity.
 6. Thesensor of claim 1 wherein the gas enters a front surface of the firstelectrode, the sensor further comprising a substantially rigidelectrolyte layer contacting a back surface of the first electrode formechanically supporting the first electrode.
 7. The sensor of claim 1wherein side surfaces of the first electrode are sealed to prevent thegas from exiting the side surfaces of the first electrode.
 8. The sensorof claim 1 further comprising a porous second electrode within thecavity, side surfaces of the second electrode being sealed to preventthe gas from entering the side surfaces of the second electrode.
 9. Thesensor of claim 8 wherein the first electrode is a working electrode andthe second electrode is a counter electrode.
 10. The sensor of claim 1wherein an adhesive is infused through sides of the first electrode toadd mechanical stability to the first electrode.
 11. The sensor of claim1 wherein the first electrode is positioned with respect to the gasopening to receive the gas into a front surface of the first electrode,and the electrolyte forms an electrolyte layer in contact with a backsurface of the first electrode, the electrolyte layer having a backsurface, the sensor further comprising: a mechanical support portion,the electrolyte layer being between the first electrode and themechanical support portion, the mechanical support portion contactingthe back surface of the electrolyte layer to, in turn, mechanicallysupport the first electrode.
 11. The sensor of claim 10 wherein theelectrode layer forms a solid or gel electrolyte layer.
 12. The sensorof claim 1 further comprising: conductors leading from the firstelectrode to metal pads external to the body, wherein the conductors areembedded within the body.
 13. The sensor of claim 12 further comprisinga metal shield layer between at least a portion of the conductors and anouter surface of the body, the metal shield layer being embedded withinthe body.
 14. The sensor of claim 12 wherein the first electrode isaffixed to the body by a conductive adhesive that also forms a gas sealaround at least a portion of the first electrode, the conductiveadhesive electrically connecting the first electrode to the conductors.15. An electrochemical gas sensor comprising: a body containing one ormore cavities; an electrolyte contained within the one or more cavities;a plurality of electrodes on the inside of the one or more cavities, theelectrodes being in contact with the electrolyte, the electrodesincluding a porous first electrode; a gas opening in the body forallowing a gas to enter the first electrode to cause a chemical reactionto occur; and the body supporting metal pads for connecting to anexternal circuit, the metal pads being coupled to conductorselectrically coupling the metal pads to the plurality of electrodes, theconductors being completely encased within walls of the body.
 16. Thesensor of claim 16 further comprising a metal shield layer encasedwithin the walls of the body, the metal shield layer being locatedbetween the conductors and at least one outer wall of the body.
 17. Asensor system comprising: an electrochemical cell having a workingelectrode, a counter electrode, and a reference electrode, wherein thereference electrode is electrically biased, and a current between theworking electrode and the counter electrode is dependent on a target gasthat permeates the working electrode; and a bias circuit coupled to thecell comprising: a controllable current source configured to output acontrollable current; an integrator coupled to an output of thecontrollable current source; and a feedback circuit coupled to an outputof the integrator, the feedback circuit supplying a first voltage to thecounter electrode and setting a bias voltage of the reference electrode.18. A method performed using an electrochemical gas sensor comprising:performing voltammetry on an electrochemical gas sensor in a nominallyuncontrolled environment within approximately one minute or less,whereby bias signals to one or more electrodes in the electrochemicalgas sensor are swept through a range of voltages to at least detect oneor more target gasses in the environment; determining one or more of therelative air humidity, air temperature, and ionic state of theelectrochemical cell from data collected during the voltammetry; andcompensating detected data relating to the target gasses from thevoltammetry for one or more of the relative air humidity, airtemperature, and ionic state of the cell as determined in the samesingle voltammetry operation.
 19. A method performed using anelectrochemical gas sensor comprising: performing voltammetry on anelectrochemical gas sensor in a nominally uncontrolled environmentwithin approximately one minute or less, whereby bias signals to one ormore electrodes in the electrochemical gas sensor are swept through arange of voltages to at least detect one or more target gasses in theenvironment; storing waveforms corresponding to the detection of thetarget gasses in the environment; and comparing the waveforms to adatabase of waveforms to identify the target gasses present.
 20. Themethod of claim 19 wherein the waveforms corresponding to the detectionof target gasses in the environment comprise overlapping gas signals,the method further comprising differentiating different gassescorresponding to the overlapping gas signals.
 21. A method of claim 19further comprising evaluating data collected by the gas sensor using aninference engine with a set of equations that identify gasses present.22. The method of claim 19 wherein the bias signals to the one or moreelectrodes are swept through the range of voltages as a continuouswaveform.